There is provided the Mobile IP as one of mobility management methods in the internet system, and Mobile IPv4 that corresponds to IPv4 is standardized by RFC3220 (IP Mobility Support). Besides, Mobile IPv6 that corresponds to IPv6, is in process of standardization by the Internet-Draft “draft-ietf-mobileip-ipv6 (Mobility Support in IPv6)” even now. It is possible for a mobile terminal to perform communication with the use of the same address even when it moves between different networks by these protocols.
In Mobile IPv6, the mobile terminal obtains prefix information of the currently connected link from the network information transmitted by the access router connected on the current link when being away from its home link. By this information the mobile terminal forms a care-of address to be used on the link. After that, the care-of address is registered on a home agent on the home link of the mobile terminal as a primary care-of address, by sending a Binding Update Message to the home agent. After receiving the Binding Update Message from the mobile terminal, the home agent creates or updates a binding cache that associates the home address with the care-of address.
The home agent, by referring to the binding cache, intercepts packets sent to the home address on the home link of the mobile terminal, then encapsulates the intercepted packets and then forwards them to the care-of address of the mobile terminal. The mobile terminal de-capsulates the packets forwarded by the home agent to be able to receive the packets sent to the mobile terminal's home address.
Further, there is provided the Fast Handover Method, that is Internet-Draft “draft-ietf-mobileip-fast-mipv6 (Fast Handovers for Mobile IPv6)”, to realize a reduction of packet loss, which is one of problems of the Mobile IP. In Mobile IP, packets for the mobile terminal's home address forwarded from the home agent to the old care-of address are discarded during the time when a new care-of address is obtained till registered in the home agent by the mobile terminal because they are impossible to be delivered to the destination address. Therefore there occurs a packet-loss. The Fast Handover Method intends to reduce packet loss by buffering and forwarding packets for the mobile terminal between the access router currently connecting to the mobile terminal and a destination access router. However, the problem is how to specify the destination access router.
As for the conventional handover methods to solve this problem, there is one for example like a description in Japanese Non-examined Patent Publication 2001-103531.
FIG. 16 is a functional block diagram of the conventional base station device.
In FIG. 16, a base station switchover information receiving circuit 1022 of the base station device 1002 obtains a base station switchover information including information on the base station to which the mobile terminal is previously connected (hereinafter referred to as “previously-connected base station”) and then a base station switchover information transmitting circuit 1023 sends it to the previously-connected base station via a wired local network connected by a network transmission/reception circuit. In the previously-connected base station, when a base station switchover information receiving circuit 1026 receives the base station switchover information from a base station to which the mobile terminal is newly connected (hereinafter referred to as “destination base station”), a terminal station information/base station information controller 1027 stores it. After that, a selection preference transmitting circuit 1024 statistically estimates a base station to become a potential movement destination from the self-station for the mobile terminal, and a base station information transmitting circuit 1025 provides it to the mobile terminal. By doing so, it is possible for the mobile terminal to determine a base station to become a handover-destination in advance, thus enabling to reduce a packet loss more certainly by used together with the Fast Handover Method.
In the mobile communication system using the conventional Mobile IP, in order to reduce a packet loss accompanied by the mobile terminal's handover, a destination access router or a base station is to be estimated and then packets are buffered and forwarded. However, it is necessary that the information for estimating a movement destination should be input into all the access routers or base stations by hand. Therefore it is a problem to reduce the configuration load especially in the case of a large-scale network environment. Besides, in Japanese Non-examined Patent Publication 2001-103531 disclosed as a way for solving the problem, there is such an improvement that information for estimating a movement destination can be automatically configured. However in this way, because movement destination is estimated by performing a statistical work based on the previous connection information, if a new base station is allocated, there is a time required before the necessary information is stored up. As a result, there is a possibility that uncertain handover is compelled in the meantime.